Nerve injury is a major risk during surgical procedures. Traditional surgical practices emphasize the importance of recognizing or verifying the location of nerves to avoid injuring them. Advances in surgical techniques include development of techniques including ever smaller exposures, such as minimally invasive surgical procedures, and the insertion of ever more complex medical devices. With these advances in surgical techniques, there is a corresponding need for improvements in methods of detecting and/or avoiding nerves.
Traditionally, the gold standard among nerve location has been direct visualization of a nerve. Direct visualization requires cutting through tissue surrounding the nerve to expose it, thereby allowing a surgeon to look at a nerve to ensure the nerve is not touched or damaged during a procedure.
Another conventional method used is nerve avoidance. By understanding human anatomy, and specifically where nerves should be within the body, a surgeon can work in the areas between the nerves, often referred to as “internervous planes of dissection;’ thereby reducing the risk of damaging a nerve during a procedure.
While direct visualization and nerve avoidance can be effective procedures, they may be impractical for certain procedures. For instance, surgery generally involves a significant amount of blood and other fluids that may obscure a surgeon's view. It may be difficult to control fluid flowing in an area of interest, thereby making it difficult to see an exposed nerve, or to determine where adjacent nerves lie. Further, the physical limitations of human anatomy make these procedures impractical for many procedures, That is, the layout of the body is something of an inexact science, and often the location of nerves, much like muscle fibers and even entire organs, can vary between patients. In addition, each of these procedures may require additional operating time, and may necessitate cutting significant amounts of unaffected tissue, resulting in an increase in pain and scarring for a patient, as well as an increased healing time.
A more recent method of nerve monitoring involves electromyography (EMG). EMG is a technique used to measure electrical activity in a motor unit during static or dynamic activity, and to evaluate the health of nerves and corresponding muscles. A motor unit generally can be described as a motor neuron and the associated muscle fibers it innervates, EMG generally includes providing an electrical stimulus to a nerve, or to surrounding tissue, and analyzing an electrical response measured through metal electrodes. EMG requires that the metal electrodes maintain a consistent electrical connection with the innervated area in order to obtain a reading. In one common approach, the metal electrodes are needles which must be driven through the skin, directly into muscle tissue. In another approach, surface electrodes are used. Surface electrodes may require significant preparation of the skin, including first cleaning the skin with alcohol, drying the skin with gauze, then shaving the skin devoid of hair and debriding the skin with pumice stone or sand paper. Once the skin has been properly prepared, EMG surface electrodes must be covered with a conductive gel to improve the electrical connection with the skin. The gel-covered surface electrodes must then be precisely placed to ensure electrical activity within the targeted muscle will be received by the electrodes.
EMG techniques have many drawbacks. EMG requires a complex, time-consuming setup procedure, and often requires a specially trained EMG technician in addition to the surgeon performing the surgery. Not only does this add to the time spent in the operating room, it can significantly increase the cost of surgical procedures. Further, surgeons are often resistant to procedures requiring the services of others. In addition to the complex setup, EMG can be an uncomfortable procedure for the patient. Needle electrodes must be driven through the skin and directly into muscle tissue. The needles may increase the risk of infection, and may lengthen the required healing time after the surgical procedure. Moreover, the needles pose an increased risk for medical professionals, due to the potential for accidental needle sticks. Debridement and skin preparation may be an irritant for patients when surface electrodes are used.
Once the electrodes are in place, it is not uncommon for them to come loose and require reattachment. Needle electrodes may be bumped during a surgery, causing them to be displaced from the target region. Surface electrodes, covered with gel, do not adhere strongly to a patient's skin and thus are prone to falling off. When electrodes lose electrical contact with a target muscle, it may not be apparent to the surgeon or EMG technician. Reattaching electrodes, and interpreting issues associated with electrodes, may further lengthen the time required for a surgical procedure, and may lead to additional frustration. Further, reattachment of electrodes during a surgical procedure may risk contamination of the sterile field. Even when EMG electrodes are properly positioned, electrical signals may be difficult to detect, and difficult to interpret. The EMG electrodes are particularly prone to interference. Accordingly, any electrical device within an operating room may affect electrode outputs. This may require a significant amount of work and interpretation to isolate the portion of readings attributable to EMG. When signals are finally received from electrodes, they are often confusing and difficult to interpret. Resulting signals are often very intricate, including various shapes, sizes, frequencies, etc. Accordingly, interpretation of EMG signals may require significant additional training for a surgeon, or may require the services of a specially trained EMG technician, to obtain meaningful information.
In addition to the foregoing, EMG systems may continually provide stimulation to a target nerve to continually monitor electrical activity. Accordingly, when using EMG systems, the muscles innervated by the targeted nerves may continually fire. This may make it difficult to properly restrain a patient, and make surgery more dangerous. It may also prompt electrodes to come loose.
Further, EMG systems which are turned on intermittently during a surgical procedure generally require a delay while a signal is detected and interpreted. This delay prolongs surgical times, and may create a period of risk and uncertainty.
These and other limitations have led to frustration and a lack of confidence in EMG techniques.